This invention relates to methods and apparatus for recovering high viscosity oil from sub-surface earth formations, and more particularly relates to improved methods and apparatus for recovering such oils by employing a large diameter shaft hole and a plurality of drill holes extending radially from the shaft hole.
Early disclosures relating to the recovery of petroleum substances by utilizing a large diameter shaft hole and a plurality of drill holes are provided in U.S. Pat. Nos. 1,520,737 and 1,634,235, and a paper published by Ranney in the Petroleum Engineer in 1939 entitled "The World's First Horizontal Hole". These publications propose the drilling of a large diameter shaft into an oil-bearing formation and then drilling radial drill holes into the formation. More recently, U.S. Pat. Nos. 4,020,901; 4,099,570; 4,099,783 and 4,160,481 provide improved systems for recovering petroleum substances employing large diameter shaft holes and radials. The above processes, however, suffer limitations relating to restrictions on the rate of introducing the injection fluid, and the overall efficiency of the oil recovery process according to the above techniques may be so poor that recovery of the oil is not economically feasible.
The publication and patents referred to above teach that steam or other heated fluids can be injected into horizontal drill holes or laterals extending from a large diameter shaft hole. The steam or other heated injection fluid is introduced via laterals into the formation in order to provide heat to the formation and thus reduce viscosity of the oil. The heated oil thereafter flows to recovery laterals, being assisted by gravity.
The above procedure is generally referred to as a "soak" operation, which should be distinguished from a "drive" operation. In a "soak" operation, the injected fluid is intended to lower the viscosity of the oil and thereby enable the oil to more freely flow toward recovery lines by gravitational forces. A soak operation may be enhanced by raising the pressure in the formation and assisting the recovery of oil by increasing the differential between the formation pressure and the pressure at the point of recovery.
It is also common practice to inject fluid into a conventional vertical well for the purpose of driving oil within a formation horizontally to be recovered by a distant vertical well. In this "drive" operation, injected fluid is intended to act as a vertical bank or wall in the formation and push or drive the oil horizontally toward the recovery well. Although the drive operation may be enhanced by injection either a heated fluid or a solvent to additionally lower the viscosity of the oil, the injected fluid is ideally acting as a piston head to move through the formation and drive the oil toward the point of recovery. Horizontal drive often results in viscous fingering and/or gravity override which are very detrimental to a horizontal drive operation. Horizontal drive operations are therefore generally concerned with the "critical velocity" of the injected fluid, so that the driving force does not break down as a result of viscous fingering or gravity override. Since the fluid injected in horizontal drive operations in preferably introduced at rates to keep the driving operation below the critical velocity, the recovery rate, and the corresponding economics of this operation, are often unacceptable. Both phenomena commonly referred to as viscous fingering and gravity override, as well as the differences between "soak" and "drive" operations, are more fully discussed in my co-pending application, Ser. No. 940,390, now U.S. Pat. No. 4,257,650.
It is also well-known to inject water via conventional vertical bore holes into the bottom of an oil-bearing formation, while producing oil from conventional vertical bore holes near the top of the formation. In this procedure, often referred to as conventional vertical flooding, the oil is displaced vertically upward by the injected water. By using conventional vertical bore holes terminating near the top of the formation and conventional vertical bore holes terminating near the bottom of the formation, it is also known that low viscosity oils can be collected by injecting a light hydrocarbon in the top of the formation while recovering the light oil from the vertical bore hole terminating at the bottom of the formation. In the latter procedure, the light oil is pushed downward by the lighter hydrocarbon. The recovery of oil utilizing conventional vertical wells and drive techniques are often poor, however, because a limited number of points of recovery are provided by the vertical wells.
These conventional techniques have several inherent problems. In order to properly "blanket" an area prior to establishing active horizontal driving displacement of the oil, the displacing fluid must be injected at a low rate. Also, if effective blanketing is to be achieved, the formation must have high permeability and the difference in density between the oil and the injected fluid should be high. If a good blanket of injecting fluid is not formed, or if a good blanket is formed but the displacing fluid is injected at too high a rate, viscous fingering can result. When this condition occurs, the injecting fluid no longer effectively displaces the oil, and the volumetric sweep efficiency of the operation falls off drastically.
In order to minimize the likelihood of viscous fingering when utilizing the prior art techniques, the injecting fluid is often introduced into the formation at low injection rates, e.g., 400 barrels of fluid per day per well. Optimum steam injection rates with vertical wells are more fully discussed in an article by Messrs. Bursell and Pittman, appearing in the Journal of Petroleum Technology, August, 1975, beginning on page 997. Low injection rates, of course, generally result in low production rates, which significantly increases the cost of production.
The problems and disadvantages of the prior art are overcome with the present invention. Novel methods in apparatus are hereinafter provided for recovering high viscosity oils from sub-surface formations, wherein a greater percentage of the oil can be recovered from the formation and can be recovered in a more efficient manner.